>>3086
It's partially about social acceptability, it's also about expectedness. A girl puts on a bathing suit expecting to be seen in it, but changes without expecting anyone to walk in.
There are contexts in which it's reversed. A girl is completely fine with a guy seeing her in her underwear if she's about to have sex with him. On the other hand, if a guy walks in on a girl just as she's finishing putting a bikini on (so nothing is showing that wouldn't be when it's fully on), she would still get mad.